1. Field of the Invention
The present invention relates to an imaging system, and more particularly to a method for image scanning device in an imaging system.
2. Description of the Prior Art
An imaging system makes use of focusing a reflected light beam through a photodetector to generate an image signal for further image processing, storing and displaying. Among various applications such as image scanners, camera recorders or facsimile machines everywhere in the modern world, there are two primary functions performed in these machines, i.e., image signal capturing and analogue-to-digital converting.
For example, the block diagram of an image scanner in the prior art is shown in FIG. 1. This system operates in the way that the light emitted by light source 10 and reflected by the document surface 11 and mirror 12 pass through lens 13 and focused by lens 13. Been focused by lens 13, the light is consequently converted to a digital signal by the analog to digital converter 15.
Thus the optical image signal is transformed into electrical image signal by CCD 14. Then the analog to digital converter (ADC) 15 convert the analog image signal to an digital image signal and then feed it to the digital calibration 16. The image acquired by scanning correction board 18 is used to generate the compensation factor that fed to digital calibration 16. Digital calibration 16 is used to provide a reference level to the digital image signal. Also, digital calibration 16 correct the defect, such as dust on the surface of optical elements, of the optical system. The output signal of the digital calibration 16 is fed to the digital signal processing (DSP) module 17 to proceed the other processes. The output signal of the digital signal processing (DSP) module 17 can be further processed, stored and displayed.
Referring to FIG. 2, the optical system is indeed an optical-energy transferring system. The system is able to transfer the optical-energy distribution of source object 212 to the optical-energy distribution of transferred image 214 through lens 213. Owing to the defects of the optical-transferring system and the diffraction of the light, the optical-energy distribution of source object 212 is distorted when source object 212 is transferred by lens 213 to transferred image 214.
In the microscopic aspect, the energy of a pixel of source object 212 is mainly transferred to a corresponding pixel and neighboring pixels, it is why the quality of an output image of the system in the prior art is degraded. Referring back to FIG. 1, thus the optical-energy distribution of the object behind document surface 11 is not the same with that of CCD 14, and the quality of the output image is degraded. Because the source signal is distorted before CCD 14, no matter what efforts the system makes, the output image is the same as the signal received by CCD 14 at best. This is also the reason why the quality of an output image of the system in the prior art can not be upgraded to a large extent.
In traditional technology, it is necessary to upgrade the specification of the optical elements in an imaging system, if the request of optical signal is critical. Whereas the optical elements with better performance always cost much, it does not meet the application of an imaging system.
The present invention relates to an imaging system, and more particularly to a method for image scanning device in an imaging system. Improving the quality of the image that produced by an imaging system, a method for enhancing modulation transfer function of an imaging system is utilized in the present invention to improve the contrast of the output image.
The first preferred embodiment of the present invention improves the contrast of image in space domain, which providing the following devices including pre-processing device, digital calibration device, modulation transfer function (MTF) enhancing device and post-processing device.
Pre-processing device for generating a digital image signal by scanning an object containing abrupt change in gray level. The pre-processing device containing a photoelectric device and an optical-transfer system, the object comprises a correction board having a surface with abrupt change in gray level. The photoelectric device is a charge-coupled device (CCD).
The pre-processing device containing an image acquiring device for acquiring an optical image as the source of the optical-transfer system a photoelectric device for converting an optical image to an electric signal and a sampling device for converting the electric signal to the digital image signal. The sampling device is an analog-to-digital converter. Digital calibration device for generating a reference level by scanning the correction board.
Modulation transfer function (MTF) enhancing device for generating a modified digital image signal by processing the digital image signal and the basic modifying vector. The modulation transfer function enhancing device is used to obtain a backward modifying vector by the product of a rearranged basic modifying vector and the digital image signal, wherein the rearranged basic modifying vector in reformed from the basic modifying vector. The other purpose of the modulation transfer function enhancing device is to generate the modified digital image signal by multiplying the digital image signal and the backward modifying vector. Post-processing device for generating image codes by processing the digital image signal.
The second preferred embodiment of the present invention improves the contrast in frequency domain, which providing the following devices including Pre-processing device, digital calibration device, Fourier transform device modulation transfer function (MTF) enhancing device and post-processing device.
The pre-processing device is the same as that mentioned in the first preferred embodiment. The output of the pre-processing device contains two parts, one is the result of scanning correction board, and the other is that of object.
The Fourier transform device is to convert the space domain signal to a frequency signal, so as to generate a basic modifying vector by extracting the amplitude and phase angle of the result of scanning the correction board. The Fourier transform device also generate a transformed digital image signal by extracting the amplitude and phase angle of the result of scanning the object, which the basic modifying vector contains a plurality of amplitude of various frequencies and a compensation angle, the transformed digital image signal contains amplitude of various frequencies.
Modulation transfer function (MTF) enhancing device is to generate a modified digital image signal by processing the basic modifying vector and the transformed digital image signal, which the transformed digital image signal is divided by the amplitude of the basic modifying vector and the phase angle of the transformed digital image signal is subtracted by the compensation angle.
The inverse Fourier transform is used to convert a frequency domain signal to a space domain signal that is the signal with degrade factor compensated.
The inverse Fourier transform device is to generate an inverse transformed digital image signal by processing the modified digital image signal, which the inverse Fourier transform device multiply the modified digital image signal by sine signals of various frequencies.
Post-processing device is to generate an image code by processing the inverse transformed digital image signal.